a) Field of the Invention
The invention is directed to a process for guiding air for cleaning semolina and to a semolina cleaning apparatus, wherein the product is cleaned via a plurality of swivelably supported sieve layers forming a cleaning section and via product discharges for the sieve tailings and is divided into the desired fractions below the sieve layers with a plurality of adjustable outlet sleeves for guiding the sieve throughs or siftings into two or more collecting troughs.
b) Background Art
Along with the roller mill and plan sifter, the semolina cleaning apparatus has developed into an apparatus of principal importance having the characteristic features of milling process technology. These characteristic features are:
an open process flow allowing the product to be viewed in the respective processing stages;
the possibility of removing product samples within critical operating stages for sensory evaluation or analytical testing;
the possibility for the miller to make corrections with respect to the adjustment of the apparatus and the product flow.
The principle of the fluid bed on one hand and classical sifting on the other are combined with a shaking movement of the sieve in the semolina cleaning apparatus.
The conventional sifting process works without air being guided through the sieve if possible. On the other hand, a fluid bed is produced with an intensive air flow in such a way that the product takes on a fluid, flowing state. The fluid bed follows very specific laws, separation by large and small parts being only the most well-known effect. The principal object of the semolina cleaning apparatus is to obtain semolina and middlings, varying granulation, and to sort out all husk parts which have a darker, often brownish color. Further, flour products with different qualitative characteristics can be sorted with the semolina cleaning apparatus.
A semolina cleaning apparatus with a completely closed system is known from the German Patent No. 29 181 from 1884. A twin ventilator sucks the air out of the upper part of the apparatus outfitted with a filter-cloth cleaning device and guides it back along the product collecting space to the lower side of the sieve. In contrast to more recent art the possibility for monitoring the operation of the semolina cleaning apparatus is very unsatisfactory, which would no longer be feasible particularly in cleaning sections having a plurality of sieve layers.
A peculiarity of mill grinding in the sense of modern high-technology milling is that, aside from the need to achieve a determined granulation, several substantial points must be taken into consideration particularly with respect to the material to be ground:
the starch granule for semolina products destined for the pasta industry must not be mechanically damaged since the pasta quality will otherwise suffer.
the gluten must not be damaged, e.g. by local heat, in the case of baking flour
the large surface area of the outer husk parts must be kept as complete as possible and separated out.
The large surface area of the husk parts, in contrast to the more cubic shape of the flour and semolina parts, made use of in the semolina cleaning apparatus for sorting them out. This is effected based on the physical forces which force the flat parts to the top in an agitated fluid bed so that the latter can be separated out as sieve tailings. Only the flour and semolina parts, depending on size, are sifted in the semolina cleaning apparatus. This process is analogous to a calibration with the aim of a determined spectrum of grain sizes.
All demands with respect to the optimal functioning of the semolina cleaning apparatus are met by the quite unique concept as developed from its origins 100 years ago. The semolina cleaning apparatus has also become a specialized machine which is used almost exclusively in the area of grain milling. Any essential change in the concept would have to result in a change in function. Consequently, recent refinements, such as closing off the entire product flow from the outside for reasons of hygiene, cannot be solved in a reasonable manner without simultaneously challenging the basic requirement for openness and accessibility of the process.